The present invention relates to an active matrix-type flat panel display.
Conventionally, active matrix-type liquid crystal display devices using amorphous silicon film are known. Further, there are known active matrix-type liquid crystal display devices which employ a crystalline silicon film and are capable of providing a higher display quality.
When an amorphous silicon film is used, there is a problem that a P-channel type thin-film transistor cannot be realized (no practical use because of low characteristics). On the other hand, in the case where a crystalline silicon film is used, a P-channel type thin-film transistor can be manufactured.
Therefore, when a crystalline silicon film is used, a CMOS circuit can be constructed by using a thin-film transistor. By utilizing this fact, a peripheral driver circuit for driving an active matrix circuit may also be constructed of a thin-film transistor.
Consequently, as shown in FIG. 10, a constitution comprising an active matrix circuit 10 and peripheral driver circuits 11 and 12 integrated on a single glass substrate or quartz substrate can be realized. Such a constitution is called as an integrated peripheral driver circuit type.
The constitution of the integrated peripheral driver circuit type has a feature of capable of downsizing the overall display device and reducing its manufacturing cost and steps.
In the case where an image with high quality is pursued, how fine gradation display can be achieved is an important factor. In general, a non-saturated region in a voltage-transmittance curve of a liquid crystal is used in case of implementing a gradation display. In other words, the gradation display is realized by utilizing the range where the optical response changes with the change of applied voltage (electric field). Generally, this method is called as an analog gradation method.
When the above analog gradation method is employed, the followings are factors leading to impair of the image quality. The main factor among them is the case in which fluctuation in voltage applied to the liquid crystal of the respective pixels becomes greater than the voltage necessary for a single gradation. In such a case, it results in the state such that an image is swaying or stripes are appeared in the display device.
The fluctuation in the voltage applied to the liquid crystals of the respective pixels is attributed to the fluctuation in the characteristics of the thin-film transistors arranged in a matrix of several hundred by several hundred. Also, in case of an integrated peripheral driver circuit type, the fluctuation in the thin-film transistor provided in the driver circuit also contributes to the fluctuation in the above voltage.
In general, the fluctuation in the characteristics of a thin-film transistor depends on numerous parameters. Accordingly, even if one of such parameters is controlled, it is still difficult to overcome the above-mentioned problems of image degradation. The problem is more serious because there is also a parameter that cannot be controlled to completely restrain the fluctuation in the characteristics of a thin-film transistor.
The present invention disclosed in this specification has an object to provide a guideline as to which parameter of a thin-film transistor should be controlled preferentially in manufacturing an active matrix-type display device.
According to the knowledge of the present inventors, the fluctuation in the drive voltage for driving a liquid crystal, which is closely related to the degradation of the image quality of a liquid crystal display device, is mostly attributed to the feed through voltage in the respective pixels.
The influence of the feed through voltage on a liquid crystal display of an active matrix type is described in Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), EID95-99, ED95-173, SDM95-213 (1996-02).
Brief explanation of the feed through voltage is given below. FIG. 11 shows the drive voltage for driving a thin-film transistor arranged in an active matrix circuit.
In FIG. 11, Vg represents a signal voltage supplied from a gate signal line to the gate electrode of a thin-film transistor. Vs represents another signal voltage supplied from a source wiring to the source region of the thin-film transistor. Further, Vd represents the waveform of the voltage applied to the liquid crystal from a pixel electrode. Incidentally, the gate signal lines and the drain lines are arranged in a matrix form.
The gate voltage Vg first rises to the ON level Vgh, then the thin-film transistor turns ON, so that the voltage signal supplied from the source signal line may be applied to the liquid crystal.
Even after the gate voltage Vg is lowered to the OFF level Vgl, the electric field is succeedingly applied to the liquid crystal by the charge stored in the liquid crystal and the auxiliary capacitance.
Thus, the image information is rewritten in the pixel electrode when a pulse of the next gate voltage Vg is inputted into the gate electrode. That is, the thin-film transistor is turned ON again when a pulse of the next gate voltage Vg is inputted into the gate electrode, and the charge corresponding to new Vs flows into the pixel electrode.
Generally, to prevent the degradation of liquid crystal, an AC voltage represented by Vsigc.+-.Vsig is used for the voltage Vs. In this case, Vsigc represents the center voltage, and Vsig represents the image signal voltage. Also, the value of Vsig corresponds to the gradation.
When driving such a thin-film transistor, the fall voltage of the gate voltage Vg in switching the thin-film transistor from an ON state to an OFF state causes the fluctuation of drain voltage through the parasitic capacity between the gate and the drain. This fluctuation in voltage is the feed through voltage (.DELTA.Vs).
FIG. 11 shows the influence of the feed through voltage (.DELTA.Vs). The feed through voltage (.DELTA.Vs) can be expressed by the following expression (1): EQU .DELTA.Vs=1/Ct[Cgd.multidot..DELTA.Vg-.intg.Idt] (1)
where, Ct represents the total pixel capacity inclusive of that of the auxiliary capacitance; Cgd represents the parasitic capacitance between the gate and the drain; and .DELTA.Vg is the fluctuation amount in the gate voltage. In case of FIG. 11, .DELTA.Vg is expressed by .DELTA.Vg=Vgh-Vgl.
The term expressed by fldt shows the influence of a current flowing between the source and the drain ascribed to the deformation in the waveform of the signal voltage supplied by the gate signal line.
Referring to FIG. 10, the signal waveform propagated through the gate wiring results in a distorted waveform 13 due to the poor characteristics of the gate driver circuit. The distortion of the signal waveform 13 is also affected by the time constant which depends on the product of the resistance of the wiring and the capacitance of the wiring. However, in case that a material of low resistance, such as aluminum, is used for the wiring, the driving force of the driver circuit becomes dominant on the waveform.
When the thin-film transistor in the active matrix region is driven by such a distorted waveform 13 as shown in FIG. 10, a predetermined time period is necessary to completely turn OFF the thin-film transistor. During this predetermined period of time, furthermore, the current flows in a direction of correcting the feed through voltage.
The term expressed by .intg.Idt in expression (1) gives the total quantity of this current.